Smoking was banned from Sunday in all public buildings in England, including pubs, movie theaters, shopping malls -- and even Buckingham Palace.
England was the final part of the United Kingdom to broadly prohibit smoking, with Wales and Northern Ireland instituting such a ban in April and Scotland last year. The Republic of Ireland made the move three years ago.
"Only by tackling the causes of illnesses will we be able to improve health inequalities and save lives," said newly appointed Health Secretary Alan Johnson. "A smoke-free country will improve the health of thousands of people, reduce the temptation to smoke and encourage smokers to quit."
International travelers were warned as their flights landed of the new rules in effect and of the $100 penalty for violations. The message was repeated on trains, on posters and in TV ads.
The decision to ban smoking was widely supported, but has also elicited vociferous objections from many who say the government has gone too far.